1. Technical Field
The present invention relates to photovoltaic devices, and more particularly to tandem devices and methods which employ different processes between tandem cells of the devices to improve performance.
2. Description of the Related Art
Solar cells employ photovoltaic cells to generate current flow. Photons in sunlight hit a solar cell or panel and are absorbed by semiconducting materials, such as silicon. Electrons gain energy allowing them to flow through the material to produce electricity.
When a photon hits silicon, the photon may be transmitted through the silicon, the photon can reflect off the surface, or the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure. When a photon is absorbed, its energy is given to an electron in a crystal lattice. Electrons in the valence band may be excited into the conduction band, where they are free to move within the semiconductor. The bond that the electron(s) were a part of form a hole. These holes can move through the lattice creating mobile electron-hole pairs.
A photon need only have greater energy than that of a band gap to excite an electron from the valence band into the conduction band. Since solar radiation is composed of photons with energies greater than the band gap of silicon, the higher energy photons will be absorbed by the solar cell, with some of the energy (above the band gap) being turned into heat rather than into usable electrical energy.
A solar cell may be described in terms of a fill factor (FF). FF is a ratio of the maximum power point (Pm) divided by open circuit voltage (Voc) and short circuit current
            (              J        sc            )        ⁢          :        ⁢                  ⁢    FF    =                    P        m                              V          oc                ⁢                  J          sc                      .  The fill factor is directly affected by the values of a cell's series and shunt resistance. Increasing the shunt resistance (Rsh) and decreasing the series resistance (Rs) will lead to a higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum. The increased efficiency of photovoltaic devices is of utmost importance in the current energy environment.